A description of the molecular biology of vitamin B6 compounds is the goal of this work. This means, in more specific terms, both a definition of how cells regulate the concentrations of the various forms of vitamin B6 and a description of any roles played by vitamin B6 compounds in macromolecular processes. In particular the effect of isoniazid resistance in E. coli will be studied since we have found that this resistance is accompanied by increased sensitivity to ultraviolet light, decreased growth rate, increased amounts of pyridoxiamine in the cell and decreased cell division. A vigorous attempt will be made to identify which macromolecular process it is that appears to correlate best with the excessive vitamin B6 in the cell. The possibility that the isoniazid resistance site is identical with the pleiotropic site coding for resistance to colicin E2 will be investigated. Related problems which will receive considerable effort will be the mechanism of control of derepression of pyridoxine biosynthesis by threonine, the pathway by which E. coli incorporates glycolaldehyde into pyridoxine and the differences in serine metabolism between E. coli B and E. coli K-12.